Changes in Body Composition and Liver and Kidney Aging
Changes in body composition are primarily characterized by a gradual loss of skeletal muscle and an increase in body fat, although the latter is more prominent in women.
Basal metabolism declines with age, with most of the decline accounted for by the change in body composition. A reduction in total body water reflects the reduction in cellular water that is associated with a loss of muscle and an increase in adipose tissue.
Aging causes a small decrease in plasma albumin levels and a small increase in α1-acid glycoprotein, yet the effect of these changes on drug protein binding and drug delivery appears to be minimal.
Liver mass decreases with age and accounts for most, but not all, of the 20% to 40% decrease in liver blood flow. Even in very old individuals, liver reserve should be more than adequate.
Renal cortical mass also decreases by 20% to 25% with age, but the most prominent effect of aging is the loss of up to half of the glomeruli by age 80 years.
The decrease in the glomerular filtration rate (GFR) after age 40 years typically reduces renal excretion of drugs to a level so that drug dosage adjustment becomes a progressively important consideration beginning at approximately age 60 years. Nevertheless, the degree of decline in GFR is highly variable and is likely to be much less than predicted in many individuals.
Aged kidneys do not eliminate excess sodium or retain sodium when necessary as effectively as kidneys in young adults. Fluid and electrolyte homeostasis is more vulnerable in older patients, particularly when the patient has acute injury or disease, and eating and drinking become more of a chore.
Aging is associated with decreased insulin secretion in response to a glucose load. It is also associated with increased insulin resistance, particularly in skeletal muscle. (Even healthy elderly patients may require insulin therapy more often perioperatively than young adults.)
Brain mass begins to decrease slowly starting at approximately age 50 years and declines more rapidly later such that an 80-year-old brain has typically lost 10% of its weight.
Neurotransmitter functions suffer more significantly, including dopamine, serotonin, γ-aminobutyric acid (GABA), and especially the acetylcholine system (which is connected to Alzheimer disease). Nearly half of patients age 85 years and older demonstrate significant cognitive impairment.
Some degree of atherosclerosis appears to be inevitable.
Contrary to prior belief, the aged brain does make new neurons and is capable of forming new dendritic connections.
There is an approximate 6% decrease in the minimum alveolar concentration per decade after age 40 years. For many intravenous (IV) anesthetic agents, the same brain concentration produces approximately twice the effect in an older person than a young adult.
Age is a major risk factor for postoperative delirium and cognitive decline.